1. Field of the Invention
The present invention relates to a method of processing seismic data and provides a technique for computing a stacked line by interpolation between known stacks.
2. Description of the Related Art
Seismic data are collected using an array of seismic sources and seismic receivers. The data may be collected on land using, for example, explosive charges as sources and geophones as receivers. Alternatively, the data may be collected at sea using, for example, airguns as sources and hydrophones as receivers. After the raw seismic data have been acquired, the reflected signals (known as traces) received by each of the receivers as a result of the process of actuation of a seismic energy source are processed to form a subsurface image. The processing includes the steps of accounting for the separation (known as offset) between sources and receivers and summing related traces together to increase signal/noise ratio (a process known as stacking).
FIG. 1 of the accompanying drawings schematically illustrates an idealised source and receiver arrangement arranged along a line. First, second and third sources 1, 2 and 3, respectively, cooperate with first, second and third receivers 4, 5 and 6, respectively. The sources and receiver are arranged about a common midpoint (CMP) 7 for the source/receiver pairs 1, 6; 2, 5; 3, 4. Seismic energy produced from the actuation of each of the sources 1, 2 and 3 is reflected from partial reflectors such as 9 and received by each of the receivers 4, 5 and 6. The travel time of the energy from a source to a receiver increases with increasing distance (offset) between the source and the receiver. The travel time is also a function of the depth of the reflectors and of the velocity of propagation of the signal within the subsurface formations.
FIG. 2 of the accompanying drawings illustrates the travel time for the situation shown in FIG. 1, as the offset increases. The round trip travel time with respect to offset for each of the reflectors defines a curve. In this simplified situation, the curve can be accurately defined by:t2 (offset)=(offset)2/(velocity)2+t2 (zerooffset)where t is the round trip travel time, offset is the distance between source and receiver and velocity is the speed of propagation of seismic signals within the subsurface formations.
During the processing of the seismic survey data, the traces are assigned to their respective common midpoints such that the geology beneath the line of sources and receivers can be probed at a plurality of positions. A velocity analysis is then performed for each common midpoint and indeed for each reflector 9. This is achieved by specifying a range of hyperbolas, as defined in the above equation, related to a range of velocities and computing the reflection amplitude along all specified hyperbolas. The seismic traces for a plurality of offsets are then converted in accordance with the hyperbolas to equivalent traces having zero offset and the traces are then summed (stacked). The resulting amplitudes at zero offset are examined to determine which hyperbola gives the best result for each of the reflectors of each common midpoint. FIG. 3 of the accompanying drawings shows a typical example of velocity analysis at point i, where the velocity function selected by the user varies between a range of known velocities functions.
Once a velocity function has been analyzed for a common midpoint, the seismic data related to the common midpoint are then corrected to zero offset according to the previous equation and then stacked for that particular common midpoint. The stacked trace has an improved signal-noise ratio compared to the traces recorded at the receivers. That process, repeated at each of the common midpoints of the line, produces a stacked seismic line that gives an indication of the geology of the line. The quality of the stacked line is directly related to the quality of the velocity field used for stacking. Stacking a line is a CPU intensive process that necessitates the use of large and powerful machines, especially if it is to be done in real time.